Motor pylons for a kite and airborne power generation system using same

ABSTRACT

A motor pylon system adapted for use with an airborne power generations system is disclosed. The pylons may support turbine driven generators for wind based electrical power generation which also function as electric motors in some aspects. The pylons may be designed to provide side force useful for turning a tethered flying wing flying in a circular cross wind flight path. The pylons may be designed to minimize air flow disruptions over the main wing.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. patent application Ser. No.13/733,125, filed on Jan. 2, 2013, which claims priority to U.S.Provisional Patent Application No. 61/582,408 to Vander Lind et al.,filed Jan. 2, 2012, both of which are hereby incorporated by referencein their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Contract NumbersDE-AR0000122/AR0000243 awarded by Advanced Research ProjectsAgency-Energy (ARPA-E). The government has certain rights in theinvention.

BACKGROUND

Field of the Invention

The present invention relates to a system and method of flying tetheredflying vehicles.

Description of Related Art

Crosswind kite systems comprising tethered wings (kites) can extractuseful power from the wind for purposes such as, for example, generatingelectricity, lifting or towing objects or vehicles, etc. To provide oruse consistent power, it may be desired to fly the kite in repeatingtrajectories (i.e., a limit cycle). It may also be desired to maintainthe kite aloft and flying consistent trajectories during a large rangeof environmental conditions such as high wind speeds, large gusts,turbulent air, or variable wind conditions. However, take-off andlanding of such kites can present difficulties, as the kites may not bewell adapted for landings similar to that of an aircraft. Therefore, amode of operation is desired so that a kite system can take-off, land,and operate safely in high and changing winds.

DETAILED DESCRIPTION

A motor pylon system adapted for use with an airborne power generationssystem is disclosed. The pylons may support turbine driven generatorsfor wind based electrical power generation which also function aselectric motors in some aspects. The pylons may be designed to provideside force useful for turning a tethered flying wing flying in acircular cross wind flight path. The pylons may be designed to minimizeair flow disruptions over the main wing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a kite system flying according to some embodiments ofthe present invention.

FIG. 2 illustrates a flying kite according to some embodiments of thepresent invention.

FIG. 3 illustrates a wing mounted pylon with turbine driven rotorsmounted thereon according to some embodiments of the present invention.

FIG. 4 illustrates a pylon with turbine driven rotors mounted thereonaccording to some embodiments of the present invention.

FIG. 5 illustrates the relative locations of rotor axes and rotormounting according to some embodiments of the present invention.

FIG. 6 illustrates pylon airfoil cross-sections according to someembodiments of the present invention.

FIG. 7 illustrates pylon and wing spar structures according to someembodiments of the present invention.

FIG. 8 illustrates lower pylon geometries according to some embodimentsof the present invention.

FIG. 9 illustrates a wing spar according to some embodiments of thepresent invention.

FIG. 10 is a side view of a pylon according to some embodiments of thepresent invention.

DETAILED DESCRIPTION

A motor pylon design for a kite system is disclosed. A kite systemcomprising motors or motor/generators may be used for a number ofpurposes. For example, a kite system comprising rotors andmotor/generators might be used for extraction of power from the wind,might be used for towing of a vehicle, might be used for surveillance,or might be used as a communications relay. A kite system of this typecan be launched and landed from a ground station in a hovering mode offlight, in which the kite hovers under thrust from on-board rotors,while the tether attaching the kite is reeled in or out from the groundstation. The on-board rotors, as referred to herein, may be adapted fordual function. When providing thrust, the rotors are viewed as motordriven propellers. When being used to convert wind energy intoelectrical energy, the rotors are viewed as turbine driven generators.When operating, the kite either flies stationary in the wind, in thematter of a traditional kite, or flies in a crosswind flight patternalong a flightpath, generally at a speed which is a high multiple of theambient wind speed. When flying in a crosswind flight pattern, thestability, controllability, and balance of forces on the kite system aresubstantially improved by use of a tail, much in the manner of atraditional aircraft. In some embodiments, the kite system is used togenerate power in the crosswind mode of flight, and on board rotorswhich are used to provide thrust during takeoff and landing, or duringlulls in wind are operated at a lower RPM and used to turn themotor/generators to produce power. There is a strong incentive to makethe main wing of such systems operate at a high coefficient of lift andhave a high aspect ratio, as the performance of the system is describedby the simplified performance metric P:

P is proportional to CL^3/CD^2. (where Cl is the coefficient of lift andCD is the coefficient of drag)

In order to have pitch control in hover through changes in thrust on thevarious rotors, and in order to keep the wake of the rotors off the mainwing, thereby reducing disturbances to flow over the main wing which mayreduce coefficient of lift, the rotors and motor/generators are placedsubstantially above and below the main wing. In addition, to counter themass of the tail, which is desired for stability in flight, and toreduce the impact of flow over the main wing on the pressuredistribution across the swept area of the rotors, the rotors andmotor/generators are located substantially in front of the main wing.Additionally, it is desirable to generate a variable level of side(lateral or along-span) aerodynamic force with the kite in order tocounter the gravitational, centripetal, and aerodynamic forces causingthe kite to deviate from the desired flightpath. It may be desirable touse the area of the vertical sides of the pylons in order to generate acomponent of the force required to turn the wing when flying insubstantially circular flight paths, and thus to allow the wing tooperate at variable levels of sideslip. Aspects of this type ofoperation are seen in U.S. patent application Ser. No. 13/288,527 toVander Lind, which is hereby incorporated by reference in its entirety.In some embodiments of the present invention is a design for a motorpylon which does not interfere with the lift-generating capacity of themain wing of the kite, which integrates structurally with the main wingof the kite, which generates significant aerodynamic side-force, andwhich has these properties at varying angles of sideslip (e.g. from −5to +5 degrees or from −10 to +10 degrees of sideslip).

In some aspects, the present invention comprises lower and upper pylonsegments, connected by a blended joint. The lower pylon attaches to thelower surface of the main wing, while the upper pylon attaches to theupper surface of the lower pylon. The foremost attachment point of theupper pylon is near the leading edge stagnation point of the main wing,such that there is little interference between the pylon and the uppersurface of the main wing. Furthermore, the blend from lower to upperpylon tapers to a narrow width, and the chord of the upper pylon issubstantially smaller than the maximum chord of the lower pylon.References to the chord of the pylon refer to the length of the sectionin the yaw axis. In some embodiments, the blend between lower and upperpylons is contoured such that the mean streamlines about the main wing,when projected onto the surface of the pylon, follow smooth contourswith decreasing curvature far from the leading edge of the pylon blend.In some embodiments, the blend between lower and upper pylons may becontoured such that the streamlines passing over the upper part of thelower pylon segment cross over to the upper surface of the main wing andflow at a small angle to streamlines passing over the upper pylonsegment about the stagnation point on the main wing proximal to thepylon.

FIG. 1 is a diagram depicting a kite system according to someembodiments of the present invention. A kite system 100 comprises atether 103 that connects the kite 101 to the ground station 102. Thekite 101 flies along the flightpath 104 at a high multiple of the speedof the wind 122 during normal operation in a cross wind flight path 104.In some embodiments, to launch and land, the kite 101 hovers underthrust from the rotors 109, which are controlled by an automatic controlsystem. Aspects of the take-off and landing of the kite system may beseen in U.S. patent application Ser. No. 13/070,157 to Vander Lind,which is hereby incorporated by reference in its entirety. To providepitch control authority, some of the rotors 109 are distributed abovethe center of gravity of the kite 101 and some are distributed below thecenter of gravity of the kite 101, as viewed in the typical aircraftbuild reference frame. In some embodiments, the tail 106 comprises ahorizontal element which is located substantially above the kite centerof gravity, and which rotates 90 degrees pitch down during hover, toboth reduce the pitching moment on the kite 101 due to the wind 122 onthe tail 106, and to stabilize the kite 101 in pitch. The kite 101further comprises a main wing 105. In embodiments in which kite system100 is used to generate power, the main wing 105 is used to generatesubstantial lift, such that the kinetic energy available in the wind istransferred into the kite 101 in the same manner as the tip of a windturbine blade.

FIG. 2 is a diagram depicting an embodiment of a kite 201, such as akite 101 comprised by the kite system 100 depicted in FIG. 1. The kite201 comprises a main wing 205, which generates substantial lift duringoperation of the kite 201 along its flightpath (e.g. flightpath 104). Insome embodiments, the main wing 205 comprises a trailing element 207which increases the maximum coefficient of lift which may be generatedby the main wing 205. The kite 201 further comprises a tail 206, whichboth counters the pitching moment generated by the main wing 205 andtrailing element 207, and increases the pitch stability, yaw stability,and coupled stability of the kite 201. However, the tail 206 has mass,which suggests that, to locate the kite center of mass 223 at its targetlocation near the quarter chord of the main wing 205, a countering massmust be located forward of the wing 205. The rotors 209, along with themotor/generators which drive the rotors 209, are located in front of themain wing 205 such that their mass counters that of the tail 206. Therotors 209 and the associated motor/generators are attached to the wingthrough pylons 212. The pylons 212 comprise upper pylons 211 and lowerpylons 213. As the main wing 205 may be operated at a high coefficientof lift, it is designed with a specific distribution of pressure alongthe chord of the wing 205 such that lift remains high, drag remains low,and the wing 205 does not prematurely stall. As the coefficient ofpressure over the surface of the wing 205 derives from the shape of thewing 205, modifications of the wing 205 have the potential to change thepressure distribution and affect stall. Furthermore, as the coefficientof lift of the wing 205 may be high in some embodiments (e.g. above 0.7as referenced to the wing area of the wing 205), air on the bottomsurface of the wing 205 is moving slowly and has a lesser effect on thepressure distribution about the wing 205 due to shape changes. Thus, thelower pylon 213 attaches to the bottom surface of the wing 205 to takeadvantage of this lower sensitivity of the bottom surface of the wing todisturbance, and the upper pylon 211 attaches to the top of the lowerpylon 213, as opposed to the top surface of the wing 205.

The kite 201 may operate for a long period of time. As such, the rotors209, wing 205, tail 206, and motor pylons 212 must be subjected to lowtime varying aerodynamic stresses to reduce structural fatigue. Therotors 209 are located substantially in front of the wing 205 such thatthe change in pressure about the wing 205 as the wing 205 generates liftdoes not substantially impact the flow entering any portion of the sweptarea of the rotors 209. The rotors 209 are located substantially aboveand below the main wing 205 such that the wakes of the rotors 209 do notimpinge on the main wing 205 during normal operation of the kite 201,thereby avoiding an increase in turbulence impinging on the wing 205.

FIG. 3 is a diagram a motor pylon 312 and the manner in which it isaffixed to a main wing 305 according to some embodiments of the presentinvention. A lower pylon 313 is affixed to the lower surface 314 of themain wing 305, and an upper pylon 311 is affixed to the leading portionof the lower pylon 313. A junction 316 joining the upper pylon 311 andthe lower pylon 313 is shaped to follow the mean streamlines over thewing 305 in the normal operating condition of the wing 305. In someembodiments, sharp corners along junction blends, such as the junctionblend 317, are incorporated such that in normal operating conditions thedefining contours of the junction blend 317 runs along the direction ofmean flow during normal operating conditions, but at low angles ofattack, flow crosses the junction blend 317 at an angle and, due to thesharpness of the junction blend 317, detaches or separates. In someembodiments, a sharp junction blend is incorporated to increase drag atlow angles of attack, such that, in cases where the flight speed of thekite and tension on the tether 102 must be limited, the kite may produceincreased drag at low angles of attack. Increased drag may be a desiredcharacteristic when flying at high wind speeds.

In some embodiments, the pylon 312 comprises asymmetric, camberedairfoils oriented at some angle of inclination about the pylon span,relative to the mean oncoming flow direction. In such cases, theaerodynamic force generated by the pylons 312 produce a side force onthe kite (e.g. kite 101), which accelerates the kite around itsflightpath (e.g. flightpath 104). In some embodiments, the trailing edgeof the pylon 312 extends behind the trailing edge of the main wing 305,and attaches to and supports the trailing elements 307. In someembodiments, just the lower pylon 313 extends beyond the trailing edgeof the main wing 305. In the depicted embodiment, the pylon 313comprises an airfoil with thickness of 15 percent of local chord, andcamber of 4 percent of local chord, and is rotated six degrees relativeto the design oncoming flow case. Thus, the pylon 312 generates a sideforce during the normal flight condition of the kite. The upper pylon311 is also cambered 4 percent of chord, but is only inclined 3 degreesfrom the typical oncoming flow. As a strong wake from the upper pylon311 would have the potential to impact the attachment of flow on theupper surface of the wing 307, the upper pylon is both smaller in chordand inclined at a lower angle than the lower pylon 313, such that theupper pylon 311 contributes a smaller portion of the side force than thelower pylon 313.

Junction 316 tapers from a wider cross-section at the top of the lowerpylon 313 to a smaller cross-section with a pointed trailing tip 315 atthe bottom of the upper pylon 311. The pointed tip 315 is located nearthe leading edge of the main wing 305, and the upper pylon 311 does notattach to the wing 305 over a large portion of the upper surface of thewing 305. In some embodiments, the pointed tip 315 is located at or nearthe stagnation point of the wing 305 during normal flight conditions.The stagnation point, of the point of flow stagnation on the leadingedge of the wing 305 moves as the angle of attack of the wing 305changes.

In some embodiments, a landing gear extension 319 extends below thebottom of the lower pylon 313, such that landing gear may attach to thebottom of the pylon 312 and clear the rotors for landing, with tetherdetached, in the manner of an aircraft. Aircraft type landing gear ofthis type are used in some embodiments which land in the manner of anaircraft in the event of a rotor, motor/generator, or power systemsfailure.

FIG. 4 is a diagram depicting a motor pylon 312, the manner in which themotor pylon 312 is affixed to the main wing 305, and the manner in whichthe motors and/or generators 318 are attached to the upper pylon 311 andthe lower pylon 313 according to some embodiments of the presentinvention. The lower pylon 313 is affixed to the lower surface 314 ofthe main wing 305, and the upper pylon 311 is affixed to the leadingportion of the lower pylon 313. The junction 316 is shaped to follow themean streamlines 420 under the wing 305 in the normal operatingcondition of the wing 305, which are seen below the stagnation point421.

In some embodiments, the pylon 312 comprises an asymmetric, camberedairfoil mounted at some angle of inclination relative to the pylon span(angle of sideslip relative to the main wing, such that the pylon 312is, in normal operation, generating lift primarily in the same directionin at all pointe around the flightpath). In such cases, the aerodynamicforce generated by the pylons 312 produces a side force on the kite(e.g. kite 101), which accelerates the kite around its flightpath (e.g.flightpath 104) when flying in a circular flight path, for example. Inthe depicted embodiment, the upper pylon 313 comprises an airfoil withthickness of 15 percent of local chord, and camber of four percent oflocal chord, and is rotated six degrees relative to the design oncomingflow case. Thus, the pylon 312 generates a side force during the normalflight condition of the kite. In such embodiments, air on the pressuresurface of the lower pylon 313 is moving slowly and has a lesser effecton the pressure distribution about the lower pylon 313 due to shapechanges. The pressure surface is what would be viewed as the bottom ofan airfoil in a horizontal configuration. Thus, the motor/generator 318attaches to the pressure surface of the pylon 313 and may protrude ontothe other (suction) surface of the pylon 312 to a lesser extent. Themotor/generator 318 is oriented such that its rotation axis is parallelto the oncoming airflow and the disc of any propeller 309 attached tothe motor/generator 318 is roughly normal to the oncoming airflow.

In some embodiments, the upper pylon 311 is also cambered 4 percent ofchord, but is only inclined 3 degrees from the typical oncoming flow. Asa strong wake from the upper pylon 311 would have the potential toimpact the attachment of flow on the upper surface of the wing 305, theupper pylon 311 is both smaller in chord and inclined at a lower anglethan the lower pylon 313, such that the upper pylon 311 contributes asmaller portion of the side force than the lower pylon 313, and thepylon junction 316 tapers from a wider cross-section at the top of thelower pylon 313 to a smaller cross-section with a pointed tip at thetrailing edge attachment point 315, at the bottom of the upper pylon311. In such embodiments, air on the pressure surface (the surfacetoward the direction of the side force) of the upper pylon 311 is movingslowly and has a lesser effect on the pressure distribution about theupper pylon 311 due to shape changes. However, the upper pylon 311 ismuch thinner and has a significantly smaller chord than the lower pylon313, thus has less space for attachment of the motor/generator 318 onany pressure or suction surface. Thus, the motor/generator 318 attachesto the top surface of the pylon 311, thereby reducing airflowinterference and potentially increasing side force capability. Thetrailing edge attachment point 315 is located near the leading edge ofthe main wing 305, and the upper pylon 311 does not attach to the wing305 over a large portion of the upper surface of the wing 305. In someembodiments, the trailing edge attachment point 315 is located at ornear the stagnation point of wing 305 during normal flight conditions.The motor/generator 318 is oriented such that its rotation axis isparallel to the oncoming airflow and the disc of any propeller 309attached to the motor/generator 318 is roughly normal to the oncomingairflow.

In some embodiments, the pylon 312 comprises symmetric, uncamberedairfoils mounted at zero angle of sideslip relative to the mean oncomingflow direction. In such cases, no aerodynamic side force is generated bythe pylons 312 on the kite (e.g. kite 101). In such embodiments, themotor/generator 318 may be mounted such that its cowling may protrude onboth surfaces of the upper pylon 311 and/or the lower pylon 313 equallyor unequally.

FIG. 5 is a diagram depicting an embodiment of a motor pylon 312, themanner in which it is affixed to a main wing 305, the manner in whichthe motors and/or generators 318 are attached to the upper pylon 311 andthe lower pylon 313, and the manner in which the upper pylon 311 isattached to the lower pylon 313. The lower pylon 313 is affixed to thelower surface 314 of the main wing 305, and the upper pylon 311 isaffixed to the leading portion of the lower pylon 313. The upper-tolower pylon junction 316 is shaped to follow the mean streamlines (e.g.streamlines 420) over the wing 305 in the normal operating condition ofthe wing 305. In the depicted embodiment, the upper pylon 311 is thinnerthan the lower pylon 313 and the junction 316 tapers to a smaller widthfrom the lower pylon to the upper pylon. In such embodiments, the upperpylon may be laterally offset from the centerline of the lower pylon313, such that the suction surfaces on the upper pylon 311 and the lowerpylon 313 are closely aligned to follow the natural streamlines (e.g.streamlines 420) of oncoming airflow, or such that any spar orstructural element may be correctly positioned.

In some embodiments, the motor/generator 318 mounted on the upper pylon311 may be offset towards the pressure surface potentially minimizingany loss of side-force due to interference with higher speed airflow onthe suction surface which is more susceptible to separation and loss ofattachment than the lower speed airflow over the pressure surface. Insome embodiments, the motor/generator 318 mounted on the lower pylon 313may be offset towards the pressure surface potentially minimizing anyloss of side-force due to interference with higher speed airflow on thesuction surface which is more susceptible to separation and loss ofattachment than the lower speed airflow over the pressure surface.

In the depicted embodiment, the majority of the side-force exerted onthe kite (e.g. kite 101) by the pylons 312 is provided by the lowerpylon 313. In such embodiments, the placement of the motor/generator 318is more critical for the lower pylon 313. In such cases, the lateralplacement of the motor/generator 318 on the upper pylon 311 may be suchthat it is at the same spanwise location along the main wing 305 as themotor/generator 318 on the lower pylon 313. This reduces angularaccelerations of the kite (e.g. 101) along axes other than thosespecifically intended, in case the motor/generators 318 are used forattitude control by changing the thrust or drag or torque to differentextents for the motor/generators 318 at various positions relative tothe kite's center of mass.

FIG. 6 is a diagram depicting the cross-sections and alignment ofcomponents of an embodiment of a motor pylon. The chord line 624 of thelower pylon, and the chord line 625 of the upper pylon are depicted. Thechord line 624 of the lower pylon is inclined 6 degrees from the meanchord direction 629 or x body axis of main wing 605, and the chord line625 of the upper pylon is inclined 3 degrees from the mean chorddirection 629 or x body axis of the main wing 305. This has the effectof reducing aerodynamic loading on the upper pylon such that the wakefrom the upper pylon incident on the top surface of the main wing 305 isweaker, and has less effect on the maximum coefficient of lift of themain wing 305. The lower pylon chord line 624 is at a greater anglerelative to the main wing 605 such that the lower pylon generatesgreater aerodynamic loads creating a larger side-force on the kite (e.g.kite 101), providing acceleration to turn the kite along its flightpath(e.g. flightpath 104). The trailing edge of the bottom profile 626 ofthe upper pylon attaches to the main wing 605 at a trailing edgeattachment point 615, which is also the foremost attachment point of themotor pylon to the main wing 305.

In some embodiments, the upper pylon profile 626 and the lower pylonprofile 628 are symmetric sections. For example, in some embodiments,symmetric profiles might be used if the target flightpath is a figureeight for which the required direction of side force changes through theflightpath, or if the target flightpath is a large circle requiring asmall side-force to turn.

FIG. 7 is a diagram depicting the internal structure of an embodiment ofa motor pylon. In some embodiments, the main wing is constructed using abox type spar, in which at least two shear walls 732 connect two sparcaps 733. In these embodiments, it is desirable to attach the motorpylon 312 to the main spar 730 of the wing by way of bonded, riveted, orbolted shear interface plates 731. The shear interface plates 731 attachto the front and back of the shear walls 732, as well as to the spars orinternal structure of the pylon 712. In some embodiments, the main wingspar 730 is formed of carbon fiber, fiberglass, Kevlar, or anothercomposite material through a bladder molding process, as a singlecomponent. In some embodiments, the shear interface plates 731 areco-molded of composite materials with the main spars of the lower pylon313. In some embodiments, the shear interface plates 731 are made ofaluminum or some other metal, and are bonded or riveted to the shearwalls of the main wing spar 730. In a preferred embodiment, the mainwing spar 730 is formed by a bladder molding process as a singlecompleted piece and the shear wall interface plates 731 comprise the topends of spars reinforcing the lower pylon 313.

FIG. 8 is a diagram depicting an embodiment of a motor pylon 812, themanner in which it is affixed to a main wing 805 and the attachment ofthe upper pylon 811 to the lower pylon 813. As the main wing 805 isoperated a at a high coefficient of lift, it is designed with a specificdistribution of pressure along the chord of the wing 805 such that liftremains high, drag remains low, and the wing 805 does not prematurelystall. As the coefficient of pressure over the surface of the wing 805derives from the shape of the wing 805, modifications of the wing 805have the potential to change the pressure distribution and affect stall.Furthermore, as the coefficient of lift of the wing 805 is high (e.g.above 0.7 as referenced to the planform area of the wing 805), air onthe bottom surface of the wing 805 is moving slowly and has a lessereffect on the pressure distribution about the wing 805 due to shapechanges. Thus, lower pylon 813 is affixed to the lower surface 814 ofthe main wing 805, and the upper pylon 811 is affixed to the leadingportion of the lower pylon 813, such that the trailing edge of the upperpylon 811 ends ahead of or within close proximity to the stagnationpoint of the main wing 821.

In the depicted embodiment, the attachment of the lower pylon 813 to themain wing 805 is smoothly filleted through the pylon to wing attachment814. As air on the bottom surface of the main wing 805 is already movingslowly due to the high lift design, the presence of sharp corners canlead to the creation of thickened boundary layers due to the retardingeffect of two walls in close proximity. This thick boundary layer andaccompanying slow airflow are highly susceptible to separation whenexposed to even a small adverse pressure gradient as can be experiencedover the trailing edge of high lift devices 807. Thus the smooth pylon813 to main wing 805 junction 814 reduces the chances of flow separationover the trailing edge devices 807. A smooth attachment point for thelower pylon 813 to the lower surface of the main wing 805 also smoothespressure distribution and thus delays tripping the airflow from laminarto turbulent, thereby reduces drag and delays the loss of attachment offlow over the trailing edge high-lift device 807 of the main wing 805.

In the depicted embodiment, the trailing edge of the lower pylon 813curves outwards toward the trailing edge high-lift devices 807 beforecurving back inwards to the attachment point 814 on the lower surface ofthe main wing 805. In such embodiments, the extent of the curve back issized such that the streamlines of airflow in the wake of the lowerpylon 813 has a minimal lateral component at the stagnation point on theleading edge of the high lift device 807, thereby reducing span wiseflow on the trailing edge device 807 and associated drag and loss ofcapacity to generate lift. In some embodiments, the trailing edge of thelower pylon 813 may be extended even further back such that the trailingedge high lift device 807 may be supported directly by the lower pylon813.

In some embodiments, the pylon 812 comprises asymmetric, camberedairfoils mounted at some angle of sideslip relative to the mean oncomingflow direction. In such cases, the aerodynamic force generated by pylons812 produces a side force on the kite (e.g. kite 101), which acceleratesthe kite around its flightpath (e.g. flightpath 104). In the depictedembodiment, pylon 813 comprises an airfoil with thickness of 22 percentof local chord, and camber of four percent of local chord, and isrotated six degrees relative to the design oncoming flow case at thebottom of lower pylon 813, with thickness decreasing to 18 percent oflocal chord and rotation decreasing to 3 degrees relative to the designoncoming flow case near the attachment point of the lower pylon to themain wing 814. In such embodiments, the decrease in thickness to chordratio of upper sections of the lower pylon 813 reduces profile dragwhile maintaining a uniform minimum thickness throughout the pylon forstructural or other purposes (e.g. for electrical conduits). The upperpylon 811 is also cambered 4 percent of chord, but is only inclined 3degrees from the typical oncoming flow. As a strong wake from the upperpylon 811 would impinge upon the upper surface of the wing 805 andeffect flow attachment at that location, the upper pylon is both smallerin chord and inclined at a lower angle than the lower pylon 813, suchthat the upper pylon 811 contributes a smaller portion of the side forcethan the lower pylon 813. The upper part of the lower pylon 813 is alsorotated to only 3 degrees, resulting in a weaker wake close to the mainwing thus reducing the potential to impact the attachment of flow on themain wing 805.

In some embodiments, the upper pylon 811 and the lower pylon 813 may beswept forward, with the leading edge of the top of the upper pylon 811and the leading edge of the bottom of the lower pylon 813 locatedfurther forward toward the normal flight direction than the leading edgeof the sections of the respective pylons vertically proximal to the mainwing 805. In such embodiments, the sweep allows the placement ofmotor/generators (e.g. 418) near the top of pylon 811 and the bottom ofthe lower pylon 813 while allowing their respective rotors to be wellclear of the main wing 805 as well as allowing the kite (e.g. 101)center of gravity to rest further forward, aiding longitudinal staticstability. By locating the rotors well above and below the main wing805, and also in front of the main wing 805, the wake of the rotors doesnot interact with the boundary layer on the main wing, and the decreasedpressure on the upper surface of the main wing does not significantlyincrease the flow velocity through the lower half of the upper rotors ascompared to the upper half of the upper rotors. In the depicted andsimilar embodiments where the pylons generate aerodynamic side-force,the sweep further promotes span wise flow along the pylons from the topof the upper pylon 811 and the bottom of the lower pylon 813 towards themain wing 805, thus increasing local static pressure near the main wingstagnation point 821, thereby reducing adverse pressure gradient of theairflow over the top surface of the main wing, which reduces likelihoodof flow separation, in turn increasing maximum lift capability of themain wing 805.

In the depicted embodiment, the lower pylon 813 is longer than the upperpylon 811. In such embodiments, a landing gear may be attached to thebottom of the lower pylon 813. In such embodiments, the lower pylon 813may be swept in sections, with the lower section of the lower pylon 813swept forward to a much lower degree than the upper section of the lowerpylon 813, or not swept at all, or swept back relative to the rest ofthe lower pylon 813 to enable clearance for rotors associated with anymotor/generators (e.g. 418) that may be mounted to the lower pylon 813.

In the depicted embodiment, the lines defining the leading and trailingedges of the pylon 812 are smooth and continuous curves, reducingoccurrence of sharp shape transitions thereby reducing formation ofregions of localized flow separation and reducing drag. In someembodiments, the top of the upper pylon 811 and/or the bottom of thelower pylon 813 may be capped by dome-shaped structures to preventseparated flow at the tips of the pylon 812. In some embodiments, otherdevices such as winglets or raked wingtips or wingtip fences may be usedto cap the tips of the pylon 812.

In some embodiments, the junction between the lower pylon 813 and theupper pylon 811 near the main wing 805 is constructed with the highpressure surfaces of the lower pylon 813 and the upper pylon 811 moreclosely aligned than their respective low pressure or suction surfacessuch that a portion of the airflow that has passed over the lower pylon813 crosses upwards to flow over the top surface of the main wing 805.In the depicted embodiment, the junction between the lower pylon 813 andthe upper pylon 811 further causes the portion of the wake of the lowerpylon 813 that is flowing over the top surface of the main wing to flowat a small angle relative to the wake from the upper pylon 811. Theforward swept angle of the pylons further sheds voracity into the airbehind the pylon 812 on the top surface of the main wing 805,re-energizing the boundary layer of the upper surface of the main wing805 behind the pylon-wing junction, thus delaying onset of flowseparation behind the pylon 812 to such time when the main wing 805 isat a higher angle of attack relative to the oncoming flow, therebyincreasing maximum lift capability of the main wing 805.

In some embodiments the entire pylon 812 may be angled such that thejunction between the upper pylon 811 and the lower pylon 813 is nolonger horizontal with respect to the ground or the gravity vector ofthe Earth. In such embodiments, the angle between the spanwise vector ofthe pylon 812 and the spanwise vector of the main wing 805 is such thatthe pylon causes the least drag from the oncoming airflow at an angle ofattack for the main wing 805 that the kite (e.g. 101) is most likely toemploy during normal flight.

FIG. 9 is a diagram depicting an embodiment of the structural connectionof a motor pylon for an airborne wind turbine main wing spar 930. Inthis embodiment, the motor pylon is attached to a glove 937 which isbonded to the shear walls of the main wing 932, and to a portion of mainwing spar caps 933. The main wing spar 930 sees both large fore and aftbending moments, as well as large up and down bending moments. Thesecombined yield large stresses on each face of the main wing spar caps933 and the main wing shear walls 932 of varying amounts over time. Theglove 937 bonds to the main wing spar 930 over a surface comprising atapered laminate, with ply drops 935 reducing the thickness of thelaminate over the length of the bond. The glove 937 further comprises aprotrusion 934 which extends below the spar caps 933 in a planar mannerfrom the shear walls 933 so as to transfer load in shear from pylonattachment points 935 in shear, through the shear walls of the glove937, and further through the bond of the glove 937 to the shear walls933 of the main wing spar 930. The pylon is attached by bolts or shearpins to the attachment points 935. This allows the pylon to be quicklyattached or detached for transport, maintenance, or replacement. Theglove 937 further bonds to the lower spar cap of the main wing spar 930.This connection is made through a curved segment 938, which preventssignificant in-plane compression of extension from creating a peel forcein the bond area, but does not prevent shear loads from beingtransferred, thereby preventing peeling forces from being transmittedinto the shear interface places 931 from thrust or drag on the rotors onthe pylons.

FIG. 10 is a diagram depicting an embodiment of a pylon substantiallysimilar to that depicted in FIGS. 3, 4, 5, 6, and 7. The rotors 1009 aredepicted by the disc or plane through which they rotate. A number ofiso-pressure lines 1034 of the main wing 305 are depicted, along withthe tangents 1035 of those iso-pressure lines most closely aligned withthe rotors 1009. In many embodiments, the rotors 1009 are located suchthat the rotor discs are near tangent to the iso pressure lines in frontof and above and below the main wing, such that the rotor discs do notintersect a significant number of iso-pressure lines or cross asignificant range in incident flow velocities. The location of therotors in front of and substantially above and below the main wingresults in not only lower differential inflow velocity over variousportions of the rotor disc, but also reduced impact of the rotor wakeand expansion field on the main wing 305. Note that, in the drawing asdepicted, the rotors would intersect five or more iso-pressure lines(equally spaced in differences of pressure) if located directly above orbelow the wing, while they intersect roughly 0.1-0.5 iso-pressure linesin their depicted configuration.

In some embodiments of the kite, the lifting surfaces are comprised ofhorizontal surfaces and vertical surfaces. In the presence of relativeairflow, the horizontal surfaces produce lift on the pitch plane and thevertical surfaces produce a lifting force on the yaw plane, i.e.,aerodynamic side-force. In various embodiments, a component of the liftgenerated by the horizontal surfaces is the primary motive force ofkite. In some embodiments, the kite is rolled relative to the tethersuch that a component of the lift generated by the horizontal surfacescontributes to the turning force of the kite. In various embodiments,the lift generated by the vertical surfaces is the primary component ofturning force of the kite. In high wind flight, the vertical surfacesare used instead of horizontal surfaces to generate the primary turningforce, while the orientation of the kite is changed such that thecoefficient of lift due to the horizontal surfaces is reduced. In thismanner larger deviations in angle of attack may be tolerated on thehorizontal surfaces prior to stall or spar failure. In some embodiments,the lifting surfaces are comprised of lifting surfaces in a number ofdifferent orientations that serve the same combined purpose of thevertical surfaces and the horizontal surfaces.

In some embodiments, the parasitic and induced drag of the horizontalsurfaces and the vertical surfaces is determined by the trim angles ofattack and side-slip of the kite and by the deflections of the controlsurfaces. In some embodiments, the drag from the horizontal surfaces andthe vertical surfaces increases significantly at a range of sideslipangles that are large, which may be seen in high wind conditions,compared to the side-slip angles observed when the crosswind kite systemoperates in normal wind conditions. In some embodiments, the coefficientof lift of the horizontal surfaces decreases at a range of side-slipangles that are large compared to the side-slip angles observed whencrosswind kite systems operate in normal wind conditions. In someembodiments, the aspect ratios of the vertical surfaces are small suchthat the vertical surfaces generate a large amount of induced drag whengenerating side-force. In some embodiments, the vertical surfaces areshaped to have a low span efficiency by comprising an irregular chord,span-wise gaps, span-wise slots, or alternating trailing edgedeflections. In some embodiments, the vertical surfaces of the motorpylons have asymmetric airfoils such that the vertical surface isadapted for lift in one direction, which may be the center of a circularflight path in some aspects. In some embodiments, a subset of liftingsurfaces comprise side-slip dependent lift modifiers, which modify thelift and drag of the surfaces which comprise them. In variousembodiments, side-slip dependent lift modifiers comprise vortilators,fences, or any other appropriate lift modifiers. In some embodiments,the lift modifiers modify the stall characteristics of a subset of thelifting surfaces as a function of side-slip. In some embodiments, thevertical surfaces comprise through-wing vents or leading edge slatswhich see little airflow in normal operation but which exhibit a largethrough flow and a large drag coefficient at large side-slip angles. Insome embodiments, the vertical surfaces comprise a subset of controlsurfaces that, when deflected or actuated, increase the side-force ofvertical surfaces at a given angle of side-slip.

In some embodiments of the present invention, a pylon might comprise aNACA 2415 airfoil and have zero angle of incidence in normal powergenerating flight, producing a pylon coefficient of lift of 0.25. At anaspect ratio of 4 and span efficiency of 1, this results in acoefficient of induced drag, referenced to the pylon area alone, of0.005. If, in high wind flight, the kite is flown at an average sideslipof 7 degrees, the pylons then generate a pylon-referenced coefficient ofinduced drag of 0.08. In some embodiments, the pylons have about 0.25 ofthe area of the main wing, resulting in an increase in coefficient ofdrag of 0.02 referenced to wing area. In some embodiments, the pylonsare shaped in a manner which has a very low span efficiency. Forinstance, the pylons may incorporate large changes in chord over shortpylon-spanwise distances, or may incorporate sharp edges near the pylontips, oriented to be aligned with the flow at kite sideslips, but to bemisaligned with the flow at high sideslips. For example, the tip of thepylon may be cut off with a square end. A pylon with a vertical pylon asdescribed above offers an advantage in that induced drag issignificantly increased when the kite is flown in sideslip. As the sideslip angle is increased in flight in high winds, induced drag increases,moderating the increase in structural loading on the system due to theincrease in wind speed.

The pylon airfoil profile may also be modified to produce greaterprofile drag above a critical angle of sideslip. For example, the pylonprofile 1301 may incorporate a leading edge cuff over a portion of thespan of the pylon, as depicted in FIG. 13. A cross-sectional profile1301 of a segment of the pylon, may cover, for example, 20% of totalpylon span. The pylon may have a leading edge cuff 1302 with a sharpcurvature discontinuity, causing a separation bubble over a segment ofthe top surface of the pylon cross-section above a critical kitesideslip or critical angle of attack of the pylon cross-section relativeto the apparent wind. As the majority of the pylon still utilizes aconventional airfoil cross-section, the added separation and parasiticdrag due to the cuffed pylon segments does not dramatically affect stallangle of attack or kite handling. A pylon with profile features asdescribed above offers an advantage in that profile drag is increasedwhen the kite is flown at a significant sideslip angle. As the side slipangle is increased in flight in high winds, profile drag increases,moderating the increase in structural loading on the system due to theincrease in wind speed.

The main airfoil, in some embodiments, has an aspect ratio of 25, andoperates at a coefficient of lift of 2 in normal power generatingflight, and a coefficient of lift of at or above 0.7 in high windflight. To provide an example, this results in a coefficient of induceddrag of between 0.085 and 0.05 referenced to wing area during normalcrosswind flight, and a coefficient of induced drag of 0.006 at the lowcoefficient of lift used in high wind flight. In this example, assumethe tether has a coefficient of drag referenced to wing area of 0.05,and a parasitic and profile drag of 0.04 referenced to wing area. Thisresults in a lift to drag of 14 for the airframe, and a performancemetric (C_L^3/C_D^2) of 400.

In high winds, again neglecting the effects of flightpath geometry, theresulting lift to drag ratio at a coefficient of lift of 0.7 on the mainwing is 7, and the performance metric is 40. If, however the added pylondrag due to sideslip previously listed (0.01) and due to change inprofile (0.01) are included, the lift to drag becomes 6, and theperformance metric becomes 25. In the example given, continued flight ofcircles becomes difficult at a coefficient of lift of 1.5, due to therequirement for excessive tether roll angle in order to complete theturn (in turn due to the lower aerodynamic force available to counteractcentripetal forces). If this is taken as the minimum coefficient of liftof a kite system not incorporating aspects of the present invention inits flight, including turning with side slip, the lift to drag andperformance metric of the system are, respectively, 12.6 and 240.Aerodynamic forces increase roughly as the square on incoming windspeed.Thus, if the nominal flight example above uses full allowableflight-loads (20000 Newtons for an 4 square meter wing, for example) inwinds of 10 m/s, the example with a minimum coefficient of lift of 1.5is able to fly in winds no higher than 13 m/s, and the exampleincorporating multiple aspects of the present invention, with a minimumcoefficient of lift of 0.7, is able to flight in winds no higher than 39m/s. Although in practice embodiments of the present invention mayutilize additional features to moderate loads in high wind conditions,one can see that just this aspect allows for a 290% increase in windcapability versus just 30% without this aspect in this exemplaryembodiment.

As evident from the above description, a wide variety of embodiments maybe configured from the description given herein and additionaladvantages and modifications will readily occur to those skilled in theart. The invention in its broader aspects is, therefore, not limited tothe specific details and illustrative examples shown and described.Accordingly, departures from such details may be made without departingfrom the spirit or scope of the applicant's general invention.

What is claimed is:
 1. A motor pylon system for use on a flying device,said motor pylon system comprising: a main wing; and one or more motorpylons, said motor pylons comprising: an upper pylon; and a lower pylon,wherein said lower pylon is attached to a lower surface of said mainwing and said upper pylon is attached to said lower pylon, and wherein aleading edge of said lower pylon extends beyond a leading edge of saidupper pylon.
 2. The motor pylon system of claim 1, wherein said lowerpylon extends beyond a trailing edge of said main wing.
 3. The motorpylon system of claim 1, wherein the one or more motor pylons provideside force for turning the main wing when air flows over said main wing.4. The motor pylon system of claim 1, wherein a trailing edge of saidupper pylon meets said main wing at a leading edge of said main wing. 5.The motor pylon system of claim 1, wherein said one or more motor pylonsare asymmetric.
 6. The motor pylon system of claim 1, wherein said oneor more motor pylons have a mean lift in a given direction due togeneration of a side force in the given direction when air flows oversaid main wing.
 7. The motor pylon system of claim 1, wherein each ofsaid one or more motor pylons comprise: a first turbine driven generatorattached to said upper pylon; and a second turbine driven generatorattached to said lower pylon.
 8. The motor pylon system of claim 7,wherein a rotor coupled to the first turbine is located in front of andsubstantially above the main wing, and wherein a rotor coupled to thesecond turbine is located in front of and substantially below the mainwing.
 9. The motor pylon system of claim 1, wherein said main wing ispart of a tethered kite system.
 10. The motor pylon system of claim 1,wherein a trailing edge of the upper pylon meets or blends into the mainwing at a point near a stagnation point of the main wing.
 11. The motorpylon system of claim 1, wherein said upper pylon and said lower pylonare swept forward.
 12. The motor pylon system of claim 1, wherein ablend between the upper pylon and the lower pylon is contoured along amean flow within a flow field around the main wing.
 13. The motor pylonsystem of claim 1, wherein said main wing comprises a box-type spar, andwherein said motor pylons are attached to said main wing with shearplates bonded to a shear wall of the box-type spar.
 14. The motor pylonsystem of claim 1, wherein said upper pylon is thinner than said lowerpylon, and a junction between said upper pylon and said lower pylontapers to a smaller width from said lower pylon to said upper pylon. 15.The motor pylon system of claim 1, wherein said lower pylon is longerthan said upper pylon.
 16. A motor pylon system for use on a flyingdevice, said motor pylon system comprising: a main wing; and one or moremotor pylons, said motor pylons comprising: an upper pylon; and a lowerpylon, wherein said lower pylon is attached to a lower surface of saidmain wing and said upper pylon is attached to said lower pylon, andwherein said one or more motor pylons are asymmetric such that the lowerpylon extends further along a chord-wise direction of the main wing thanthe upper pylon.
 17. The motor pylon system of claim 16, wherein saidupper pylon comprises a first vertical airfoil element and wherein saidlower pylon comprises a second vertical airfoil element.
 18. The motorpylon system of claim 16, wherein said first vertical airfoil element ismounted at an angle such that said first vertical airfoil elementgenerates a side force in a first direction when air flows over saidmain wing.
 19. The motor pylon system of claim 18, wherein said secondvertical airfoil element is mounted at an angle such that said secondvertical airfoil element generates a side force in said first directionwhen air flows over said main wing.
 20. The motor pylon system of claim16, wherein the lower pylon of said one or more motor pylons supports atrailing element or flap of said main wing.